Common photosensitive resin printing plates which can be processed by an aqueous developer solution generally consist of a structure comprising: a support layer for maintaining dimensional precision; a layer of photosensitive resin composition comprising a mixture of a hydrophilic copolymer, a hydrophobic resin such as a thermoplastic elastomer, a photopolymerizable unsaturated monomer, a photopolymerization initiator, etc. on the support layer; and further thereon a thin flexible film layer, known as a slip layer or a protective layer, and a thin layer which can be ablated by an infrared laser.
With respect to a method of producing a printing plate from the above structure, a relief for printing is formed, when a flexible film layer is provided, by sticking a negative film thereonto, irradiating the structure with active rays through the negative film so as to selectively photo-cure specific areas of the photosensitive resin composition layer whereby an image is formed, and thereafter removing unexposed areas of the photosensitive resin composition layer with an aqueous developer solution (development).
With respect to such a photosensitive resin composition for flexographic printing, it is preferred from the viewpoint of faithfully obtaining needed images that individual components in the composition are finely and uniformly dispersed. Further, for shortening the time required for preparing a printing plate, it is preferred to increase the rate of development of the photosensitive resin composition. The photosensitive resin composition has to have satisfactory developability in an aqueous developer solution as above, but also, when made into a printing plate, high print wear characteristics of printing plate when used with an aqueous ink and also rupture resistance of the printing plate when the ink adhering to the printing plate after printing is wiped off with water or the like (plate wiping-off resistance). That is, the printing plate is required to have high water resistance and a high plate wiping-off resistance.
Various photosensitive resins which can be developed in an aqueous developer solution have been proposed in conformity with intended uses. JP-A-7-114180 proposes a photosensitive resin composition comprising a hydrophilic copolymer wherein phosphoric ester groups are contained as an essential component. An object of the invention described therein is to provide a resin composition which can be processed in an aqueous developer solution, which has photosensitivity, transparency and workability, and which can be cured into a cured product with excellent transparency and low-temperature elasticity. However, because of the use of phosphoric ester groups as an essential component of hydrophilic groups of the hydrophilic copolymer, its water resistance, plate wiping-off resistance and print wear characteristics of the printing plate from the resin composition are not necessarily satisfactory. In Comparative Example 2C of the laid-open reference, methacrylic acid is used in place of the phosphoric ester. However, the amount of methacrylic acid used is so large that its water resistance and print wear characteristics of the printing plate from the resin composition are not satisfactory.
U.S. Pat. No. 5,731,129 proposes a photosensitive resin composition comprising a hydrophilic copolymer wherein an unsaturated monomer having a carboxyl group and a conjugated diene monomer are contained as essential components while an aromatic vinyl compound and an alkyl (meth)acrylate are contained as optional components. An object of this invention is to suppress the swelling, strength drop and dimensional change by water, at the time of developing in an aqueous developer solution. However, because all of an unsaturated monomer having a carboxyl group, a conjugated diene monomer, an aromatic vinyl compound and an alkyl (meth)acrylate are not simultaneously employed in the monomers for the polymerization of the hydrophilic copolymer, the compatibility of the resin composition, developability with an aqueous developer solution and the print wear characteristics of printing resultant plates are not necessarily satisfactory.
JP-A-63-8648 proposes a photosensitive resin composition comprising hydrophilic copolymer particles crosslinked by a polyfunctional vinyl compound. An object of this invention is to suppress deteriorations of curing efficiency and relief image reproducibility by light, by using a hydrophobic resin as a matrix component and hydrophilic resin particles that have chemically and optically approximate compositions to each other. However, because a conjugated diene is not used as a monomer in the polymerization of the hydrophilic copolymer, the water resistance and print wear characteristics of the printing plate are not satisfactory. Further, because of the use of a polyvinyl compound in high proportion, satisfactory print wear characteristics cannot be attained.
JP-B-5-5106 proposes a photosensitive resin composition comprising a hydrophilic polymeric compound having carboxyl groups. An object of this invention is to suppress a decrease in resistance to a water or an alcohol base ink by using the above hydrophilic polymeric compound and a hydrophobic copolymer as a polymer component. However, because all of an unsaturated monomer having a carboxyl group, a conjugated diene-type monomer, an aromatic vinyl compound and an alkyl (meth)acrylate are not simultaneously employed in the monomers for use in the polymerization of the hydrophilic copolymer, the compatibility and developability with aqueous developer solution of resin composition and the print wear characteristics of resultant printing plates are not necessarily satisfactory.
U.S. Pat. No. 5,348,844 proposes a photosensitive resin composition comprising a hydrophilic copolymer wherein an unsaturated monomer having a carboxyl group, a conjugated diene-type monomer and a polyfunctional vinyl compound are contained as the essential components while an aromatic vinyl compound and an alkyl (meth)acrylate are contained as optional components. An object of this invention is to suppress deteriorations of elasticity, print wear characteristics and relief image reproducibility. However, because all of an unsaturated monomer having a carboxyl group, a conjugated diene-type monomer, an aromatic vinyl compound and an alkyl (meth)acrylate are not simultaneously employed, the compatibility and developability with aqueous developer solution of resin composition and the print wear characteristics of resultant printing plates are not necessarily satisfactory.
JP-A-2001-512180 proposes a photosensitive resin composition comprising a hydrophilic copolymer from ethylenically unsaturated monomers including a surfactant monomer. It is described therein that a hydrophilic polymer can be produced without employing a hydrophilic monomer as an essential raw material and by a one-stage polymerization process. However, because all of an unsaturated monomer having a carboxyl group, a conjugated diene monomer, an aromatic vinyl compound and an alkyl (meth)acrylate are not simultaneously employed in the monomers for use in the production of the hydrophilic copolymer, the water resistance and print wear characteristics of the printing plate from the resin composition are not necessarily satisfactory.
As apparent from the above, the conventional photosensitive resin compositions which can be developed with an aqueous developer solution, because of being mixtures of a hydrophilic component and a hydrophobic component as mentioned above, have posed such problems that the mutual compatibility (dispersibility) thereof is questionable, that the developability with aqueous developer solution of resin composition is low, that the printing plate prepared therefrom through exposure exhibits low print wear characteristics, low water resistance or low plate wiping-off resistance.
The technological problem to be solved in the present invention is, with a view toward solving the above problems, to provide a photosensitive resin composition which can simultaneously satisfy the following six requirements:    1) the mutual compatibility (dispersibility) of components in the photosensitive resin is high;    2) the developing time with an aqueous developer solution is short;    3) the image reproducibility of printing plate is high;    4) the water resistance of the printing plate is high;    5) the wear resistance (abrasion resistance) of the printing plate (especially upon using a water base ink) is high; and    6) the plate wiping-off resistance exhibited upon removing of the ink adhering to the printing plate is high.